In many of its applications, isoolefin copolymers, in particular butyl rubber is used in the form of cured compounds.
Rubbery copolymers of isoolefins of 4 to 7 carbon atoms, such as isobutylene, and aliphatic dienes of 4 to 14 carbon atoms, such as isoprene or butadiene, are known by the generic name of “butyl rubbers” (IIR).
Such materials usually have a long chain, linear molecular structure and a rather low level of chemical unsaturation provided by the aliphatic diene. These two characteristics contribute to their typical properties of good resistance to the degradative action of oxygen, ozone, acids, alkalis, heat and sunlight, and low permeability to gases and vapors.
The level of unsaturation of butyl rubber is normally given in mole percent, i.e., the number of double bonds per 100 monomer units. Commercially available butyl grades from LANXESS usually have between 0.7 and 2.5%. For comparison, the degree of unsaturation of natural rubber and polybutadiene rubber is 100% and for SBR over 80%.
Butyl rubber can be manufactured by copolymerizing isobutylene and isoprene in methyl chloride diluent using aluminum chloride as a catalyst. This cationic polymerization is carried out in a continuous reactor at temperatures below −90° C. A solution process is also known, with a C5–C7 hydrocarbon as solvent and an aluminum alkylhalide catalyst.
The isoprene incorporated in IIR (ca. 0.5–2.5 mol %) provides double bonds, which allow the rubber to be vulcanized with sulfur and other vulcanizing agents. Butyl rubber and its vulcanizates are characterized by impermeability to air, high damping of low frequency vibrations, and good resistance to aging, heat, acids, bases, ozone and other chemicals. These characteristics lead to the use of butyl rubber in tire inner tubes, tire curing bladders and bags, vibration insulators, roof and reservoir membranes, pharmaceutical bottle stoppers and other applications.
Unsaturated rubbers, however, are subject to oxidation by chain scission. This process normally involves the unsaturated sites of the molecular chain. Therefore, a fewer number of such sensitive sites in the rubber result in improved oxidation resistance.
Butyl rubbers in its uncured state can be used in sealants and adhesives. The presence of unsaturation in the backbone of the base polymer may lead to yellowing of the adhesive and eventual crosslinking which makes the bond between the adhesive and the adhering material brittle. The higher the degree of unsaturation in the rubber, the more potential for undesirable changes.
Most applications of butyl rubber are based on its vulcanized compounds. The vulcanization or curing of copolymers of isoolefins and dienes is most commonly accomplished by sulfur curing agents, either sulfur itself or sulfur donors, in conjunction with such accelerators as thiazoles, for example, 2-mercapto benzothiazole, thiocarbamates, e.g., zinc dimethyl dithiocarbamate, thiurams, e.g., tetramethyl thiuram disulfide, and various others, for example diphenyl guanidine and aldehydeamines. Non-sulfur curing systems can also be used, for example quinoid systems involving the use of such compounds as p-quinone dioxime and p,p′-dibenzoylquinone dioxime in conjunction with activating compounds such as red lead; p-dinitroso benzene systems and resin cure systems involving the use of such compounds as polychloroprene and brominated dimethylol phenyl resins.
For some specific applications, compounds with high hardness are needed. For example, this is the case with the printing blankets for the dry offset cup, tube and lid printing markets.
Dry offset printing is similar to offset lithography. The role of a rubber blanket is to carry the image from the printing plate to the container surface. The plate has the image area raised above the surface of the plate, as in letterpress. Ink is distributed through a series of rollers and onto the raised surface of the plate. The plate transfers the image to the blanket, which then prints the entire multicolor copy on the container. One to ten colors can be printed in a single pass over the container, with all colors being applied simultaneously by the same blanket.
Varying blanket materials and blankets with different thickness are available for varying printing requirements on different products. One of these known materials for printing blankets is butyl rubber. The blanket is manufactured with a butyl rubber compound for optimum resistance to most UV and IR printing inks. For example, one type of products known in the marketplace has the rubber face purposely harder than the standard butyl and offering improved wear characteristics at higher speeds with better ink release and print quality. Such a printing blanket can be used for most applications, including lid printing and offers excellent solvent resistance.
The polymerization of diisopropenylbenzenes (DIPB's) was first reported over 45 years ago. Free radical initiated polymerizations produced crosslinked gels.
The use of an anionic technique made it possible to produce essentially linear, soluble polymer in which only one unsaturation site of each DIPB molecule was consumed. At low conversions, the aromatic ring of each pendant group carried an unreacted isopropenyl group (“Makromol. Chem.”, 183, (2787 (1982), U.S. Pat. No. 4,499,248). Branching and crosslinking could occur at higher conversions (>50%).
The cationic polymerization of DIPB's was found to produce polymers containing predominantly a polyindane structure (“J. Polym. Sci.”, 28, 629 (1958). The molecular weight increased in a stepwise manner with time and the overall process was kinetically more akin to a polycondensation than to a conventional vinyl polymerization. The continuation of the vinyl addition beyond the dimer stage led to crosslinked products.
D'Onofrio (“J. Appl. Polym. Sci.” 8, 521 (1964) demonstrated that linear, high molecular weight, soluble polyindane was produced from diisopropenylbenzenes at polymerization temperature above 70° C. using a complex Lewis acid type initiating system (LiBu—TiCl4—HCl). It was pointed out that with the use of BF3, TiCl4, SnCl4, etc., a narrow polymerization range (70–100° C.) was necessary in order that soluble polymer was obtained. At temperatures below 70° C. crosslinked products resulted. At temperatures higher than 100° C., the activity of the catalyst decreased.
Sonnabend (U.S. Pat. No. 3,004,953) claimed a direct cationic copolymerization of diisopropenylbenzenes with phenol. The process was complicated by the simultaneous occurrence of propagation and alkylation reactions, with products exhibiting branching and ultimately gelation.
H. Colvin et al. described a direct cationic copolymerization of m-diisopropenylbenzene and m-dimethoxybenzene (in “New Monomers and Polymers”, B. Culbertson and C. Pittman (Eds.), Plenum Press, New York 1984, 415–428). The dimethoxybenzene could be incorporated into the polymer backbone or as a pendant group. The most important variable in controlling the ration of mono- to dialkylated dimethoxybenzene was the catalyst. The Mw of the polymer was below 50,000 g/mol and the properties were poor.
Copolymers of p- or m-diisopropenylbenzene with styrene exhibited unusually high melting points and increased chemical and heat resistance (Brit. 850, 363). These copolymers containing at least 5% of the difunctional monomer could be useful as moulding resins, adhesives, printing inks and as additives for lubricating oils to raise the viscosity index of the oil. The preferred catalyst was a cationic catalyst.
U.S. Pat. No. 3,067,182 discloses uniform copolymers of isopropenylbenzene chloride with isobutylene could be made under cationic copolymerization conditions at temperatures below −100° C. Such copolymers could be readily crosslinked with amines or phenols or by adding a Friedel-Crafts catalyst to obtain cure by self-alkylation.
Multi-arm star polyisobutylenes were prepared by the “arm-first” method (“Macromol. Symp.”, 95, (1995) 39–56). This synthesis was accomplished by adding various linking agents (“core builders”) such as p- and m-divinylbenzene and p- and m-diisopropenylbenzene (DIPB) to living PIB+ charges and thus obtaining a crosslinked aromatic core holding together a corona of well-defined arms. The products were characterized in terms of overall arm/core composition, molecular weight and molecular weight distribution.
Star-shaped polymer, useful as viscosity modifier for lubricating oil, comprised poly(diisopropenylbenzene) as core with at least three polyisobutylene arms (EP 1099717 A). Polymerization occurred in the presence of titanium tetrachloride and pyridine (living polymerization).
Co-Pending Canadian Application ((int. docket no. POS 1153, filed September, 2003) refers to sulfur-curable copolymers of isobutylene and diisopropenylbenzene having high Mooney viscosity (ML 1′+8′@125° C.>80 units) in the absence of multiolefins. Further, this application is silent about high Shore A2 hardness of the cured compounds.
Co-pending applications CA-2,386,628 and CA-2,368,646 provide a compound containing at least one elastomeric polymer containing repeating units derived from at least one C4 to C7 isomonoolefin monomer, at least one C4 to C14 multiolefin monomer or β-pinene, at least one multiolefin cross-linking agent and at least one chain transfer agent, said polymer containing less than 15 wt. % of solid matter insoluble within 60 min in cyclohexane boiling under reflux, at least one filler and a peroxide curing system. The multiolefin cross-linking agent can be a multiolefinic hydrocarbon compound. Examples of these are norbornadiene, 2-isopropenylnorbornene, 2-vinyl-norbornene, 1,3,5-hexatriene, 2-phenyl-1,3-butadiene, divinylbenzene, diisopropenylbenzene, divinyltoluene, divinylxylene or C1 to C20 alkyl-substituted derivatives of the above compounds.
A co-pending application filed with the Canadian Intellectual Property Office on Aug. 05, 2003 under the attorney docket POS 1142 CA provides a method of improving reversion resistance of a peroxide curable polymer containing at least one polymer having repeating units derived from at least one isomonoolefin monomer and at least one aromatic divinyl monomer by polymerizing the monomers in the presence of at least one m- or p-diisoalkenylbenzene compound. The present invention does not include the presence of aromatic divinyl monomers like divinylbenzene.
However, an elastomer having repeating units derived from at least one isomonoolefin monomer, at least one multiolefin monomer, at least one diisoalkenylbenzene monomer, and optionally further copolymerizable monomers, polymerized in the absence of aromatic divinyl monomers, wherein the elastomer has a Mooney viscosity (ML 1+8@125° C. according to ASTM D1646) of less than 40 units and a Shore A hardness higher than 65 points @ 23° C. (according to ASTM D2240) is unknown.